1. Technical Field of the Invention
The present invention relates generally to a noiseless circuit structure of a gas concentration measuring apparatus equipped with a gas sensor and a heater working to heat a sensor body up to an activation temperature.
2. Background Art
Gas concentration measuring apparatuses for use in automotive internal combustion engines are known in the art which include a gas sensor outputting an electrical signal arising from a given component of gas and a measurement control circuit determining the concentration of the given component using the sensor signal. Such a type of gas concentration measuring apparatus has the gas sensor installed in an exhaust pipe of the internal combustion engine and measure the concentration of oxygen contained in exhaust emissions of the engine for use in control of the engine.
Typical gas sensors use an oxygen ion-conductive solid electrolyte material such as zirconia. For example, gas sensors are known which have a gas chamber into or from which oxygen is allowed to move from or to outside the gas sensor and a cell which is made up of a pair of electrodes affixed to a solid electrolyte body works to pump the oxygen into or out of the gas chamber. Such a type of gas sensor pumps the oxygen into or out of the gas chamber in response to application of voltage to produce a limiting current between the electrodes as a function of concentration of the oxygen. Gas sensors are known which include a plurality of cells of the above type in order to measure the concentration of NOx (nitrogen oxide), CO (carbon monoxide), and HC (hydro carbon).
The gas sensors using the solid electrolyte material need to elevate the temperature of the solid electrolyte material up to an activation temperature thereof. To this end, a heater is typically built in the gas sensor. The power supply to the heater is usually controlled using a PWM signal because of ease and high-precision of adjustment of the power supply. Gas sensors of this type, however, encounter the following drawback.
The heater is, as described above, built in the gas sensor, so that an output of the gas sensor may be sensitive to an electric current flowing through the heater due to its leakage, capacitive coupling, and induction. The inventors of this application have found that the power supply to the heater under the PWM control causes the output of the gas sensor to change sharply due to electric noises produced when the heater is switched between an on-state and an off-state.